Thermal relay



June 17, 1930. H. E. WHITE "THERMAL RELAY Filed April 5, 1928 2 Sheets-Sheet l A'TTORNEY INVENTOR Hare/d5 A/h/fe.

BY I

H. E. WHITE THERMAL RELAY June 17, 1930.

Filed April 5, 1928 2 Sheets-Sheet 2 Rh m 2 M W: m Vd T. 2 m/ A U/Pv m w 9 w 55 w 7 3 AW 8 l I 4 M .3 z; 9 2 i 4 Z M w 3%. F n 5 3 4 Patented June 17, 1930 UNITED sra'ras PATENT OFFICE HAROLD 1?. WHITE, or wILxINsBURe, :PENNsYLvANIA, ASSIGNOR .ro WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION or PENNSYLVANIA THERMAL RELAY Application niea April 5,

This invention relates to protective devices and particularly to thermal relays.

An object of the invention, generally stated, is to provide a thermal relay that shall be simple and eflicient in operation and easily manufactured and installed.

Another object of the invention is to provide a thermal relay that shall be adapted to protect electric-energy translating devices,

such as electric motors, from moderate long;

continued overloads.

A further object of the invention is to provide an adjustable thermal relay that shall be capable of instantaneous operation when electric-energy translating devices, such as motors, are subjected to abnormal overload currents causedv by the stalling of the motors.

And a still further object of the invention is to provide for the protection of electric motors against damage resulting from sudden abnormal overloads, such as may be caused by blocked rotors, and moderate overloads long continued, or a' combination of moderate and abnormal overloads.

Other objects of the invention will, in part,

be obvious and will, inpart, appear hereinafter.

The invention is disclosed in the embodiment thereof shown in the drawings and COII1- prises the structural features, the combination of elements, and arrangement of parts I that will be exemplified in the structure hereinafterset forth and the scope of the application of which will be indicated in the appended claims.

For a fuller understanding of the invention, reference may be had to the following description, taken in conjunction with the accompanying drawings, in which Figure 1 is a top plan view of a relay embodying the invention;

Fig. 2 is a front View of the device shown in Fig. 1; I

Fig. 3 is a side view of the relay illustrated in Figs. 1 and 2, and

thermal 1928. Serial No. 267,538.

Fig. 4 is aside View of a portion of the relay, illustrating the initial effect of a sudden abnormal overload on the relay.

In the drawings, a base 1 of a'thermal relay is illustrated on which a magnetic core 2 of U-shape is mounted. The core 2 may be secured to the base by means of a core bolt 3, having a tube 4 of insulating material disposed between the head of the core bolt 3 and the base of the core 2. The core bolt 3 extends through the base of the core 2 of U-shape and the base 1 and may be secured in place by means of a stud bolt 5. i

A coil 6, of edge-wound strap copper, or other suitable material, is disposed about the central core bolt 3 and held in place and secured to the base by means of terminal bolts 7 and 8 which extend through terminals 9 and 10 of the coil.

A bracket 11 is secured to the upper end of one of the legs of the core 2 by means of screw bolts 12 and 13 (see Figs. 3 and 4). The bracket 11 is preferably of brass or other non-magnetic material.

main armature 14 of substantially L-shape is provided with notches at the rear portion thereof that register with lugs 15 and 16 of the bracket 11.

In order that an electric circuit may be controlled by the relay illustrated in the drawings, a contact-bridging. member 17 is provided. The bridging member is secured to an arm 18, of suitable electric insulating material, and positioned to bridge a pair of stationary contact members 19 and 21. The contact members 19 and 21 are preferably made of graphitic-material or an alloy known as graphalloy mounted in supporting bushinfis 22 and 23 of metal, such as copper or ot er suitable material. The bushings '22 and 23 maybe secured to the ends of bolts 24 and 25 which extend through the base 1 and are suitably secured thereto by nuts and washers. I

The arm 18, to whichthe contact-bridging member 17 is secured, and the armature 1a may be yieldingly mounted on the bracket 11 by means of a bolt 27, a washer 28, a key 29, extending through the upper end of the bolt, and a spring 31 located between the washer and the arm 1-8. In order that a rolling contact may be effected between the end of the arm 18 and the main armature 1%, a rivet 32 is secured to the end of the arm 18 (see Figs. 3 and 4-).

The spring 31 presses on the arm 18, which, in turn, presses on the armature 14 to the right of the support 11, the pressure being of such value that the armature is biased towards the position shown in Figs. 3 and 4. of the drawings.

in order that the main armature 14 may be further biased towards the position shown in Figs. 3 and 4, so that the pull of the core bolt 3. when the coil 6 is energized, will not actuate the armature until a current of a predetermined value has traversed a motor, (not shown), for a predetermined length of time, a thermal element 33 is provided. The thermal element 33 comprises a strip of reversibly magnetic material having a recess 34 in one edge thereof and a rivet 35 of soft iron or other suitable magnetic material secured to the edge of the strip opposite to that in which the recess 34: is provided. The rivet 35 normally engages the armature 14. The recess causes the strip to be heated to a higher temperature at its middle portion than at its ends in order that it may be rendered nonmagnetic at the middle portion first by current flow therethrough.

The thermal element 33 is supported, at its ends, on a pair of terminal bolts 37 and 38 and is electrically connected thereto by means of nuts 39 and40 between which the strip is positioned. The bolts 37 extend through the base and may be secured thereto by nuts 41 or other suitable means.

The character of thethermal element 33 is such that, when it is normally cool, the strip is magnetic but, when heated to a temperature of a predetermined value, it becomes non-magnetic, and, when cooled to a temperature slightly below this value, it regains its magnetic properties' For this reason, strip 33 may be designated as a reversibly magnetic member. I

In the present form of the invention, it has been found that, if the strip 33 is made from nickel steelconsisting of approximately 35% nickel and iron, with traces of other elements utilized in the alloying of metal ineluded, the relay will operate accurately and satisfactorily for my present purpose. The strip will be magnetic at temperatures below approximately 150 0., but, when heated to a temperature above this value, it becomes non-magnetic and, when cooled to a temperature slightly below, it again becomes magthe I netic. An alloy having the constituents mentioned above is commonly known as invar.

In practice, the thermal element 33 and the coil 6 may be connected in series-circuit relation with each other by connecting the terminals 37 and 7 by a conductor 4A.

In order to provide a magnetic path adjacent to the core 2 of U-shape, whereby suit able operating characteristics of the relay may be obtained, tubular members 42 and 43 of magnetic material are disposed between the nuts 40 and 4.1 and coaxially of the bolts 37 and 38, respectively.

If the coil 6 and the thermal element 33 be connected in series-circuit relation with a motor, and the terminals 24 and 25 be connected in circuit with the holding coil of a circuit interrupter, (not shown), for operating a circuit breaker, (not shown), controlling the motor, the armature 1 1 will remain in the position shown in the drawings so long as the thermal element 33 is ma netic and the current traversing the motor isbelow its fullload value.

If the load on the motor is so increased that the current is caused to increase above fullload value, say 125% full load, the strip 33 will be heated to the temperature at which it becomes non-magnetic at the end of a predetermined length of time, at which time the magnetic'pull between the armature l i'and the core bolt 3 will be suflicient to attract the armature to the position shown in broken lines in Fig. 3 of the drawings. In this position, the contact-bridging member 17 is moved to a disengaged-position with respect to the contacts 19 and 21, and the circuit controlled thereby is broken and the motor caused to be disconnected from the source of electric power to which it is connected.

In order to prevent the main armature 14 from sticking to the core 2 of U-shape, a strip 45 of non-magnetic material, such as co 'aper or brass, is secured to the upper end 0 the left leg of the core 2 (as viewed from Figs. 3 and 4), whereby an air gap is maintained between the core and the armature when in its attracted position.

If the armature 14 is utilized by itself, motors may be protected from being overheated either by moderate overloads long continued or a definite abnormal overload of one particular value, that is, the relay cannot be adjusted to operate at any one of a plurality of abnormal overload. values. Therefore, in. order that the relay may be adjusted to operate instantaneously over a wide range of abnormal current values, say from 200% to 700% full-load current, such as may result from a stalled rotor or from overloads incident to starting the motor from rest under load, an auxiliary armature 46 is provided (see Fig. 4). One endof the armature 46 is sitioned between the main armature '14 an the bracket 11 and is yieldingly supported by the armature by means of a bolt 47, having screw-thread engagementwith the armature 46 and extending through an opening in the armature 14, a nut 48 and a spring 49 positioned between the nut and the main armature 14. U

In the event that the coil 6 and the thermal element 33 are traversed by a current of six or seven times full-load current of the motor to be protected, the auxiliary arn'iatiiire 46 is attracted to the position shown in Fig. 4 of the drawings, the attraction between the armature 14 and the normally magnetic member 83 being sufficient to hold the armature in the position shown in Fig. 4. The pressure exerted upon the main armature 14 by the spring 49 and the decrease in magnetic flux through the armature 14 incident to the separation of the main and auxiliary armatures cause the armature 14 to be released to the position shown in broken lines in Fig. 3, substantially instantaneously with the passing of a current of this value through the coil and the thermal element.

VVhenconditions of this kind arise, the thermal element 33 may not have been heated to the temperature at which it becomes nonmagnetic; therefore, if the armature 14 were not released therefrom immediately upon being traversed by a current six or seven times the full-load value, the motor may have been damaged by excessive heating, as a result of such current flow.

The relay herein described operates according to a definite inverse-current-time characteristic, when the motor to be protected thereby is operating under normal full-load conditions, over a range extending from zero full-load to the value of current which will cause the relay to operate instantaneously. By way of example, if the motor is subjected to 125% full-load,then the relay will operate at the end of an hour; it the load is 200%, the relay will operate at the end of one half hour, etc.

The inverse-current-time characteristic may be modified by properly proportioning the thermal element 33 and choosing a spring 31 having the proper stiffness.

By adjusting the spring pressure of the spring 49 upon the main armature 14 by means of the nut 48 and the bolt 47, the value of current at which it is desired that the relay trip out instantaneously may be varied from approximately 300% to 700% full-load value.

By means of the relay illustrated in the drawings, motors may be protected both from moderate overloads long continued and from abnormal overloads. In the latter case, the relay is. adapted to operate instantaneously upon the coil 6 and the thermal element 33 being traversed by a current of a predetermined value, even though the thermal element 33 may not have been heated to its nonmagnetic temperature.

Various modifications may be made in. the

device embodying the invention, as illustrated in the drawings and set forth in the specifi cation, without departing from the spirit and the scope thereof. It is desired, therefore, that only such limitations shall be placed thereon as are imposed by the prior art and the appended claims.

I claim as my invention:

1. A protective relay comprising an armature, a coil for actuating the armature, means for restraining the armature from movement by said coil according to an inverse-currenttime characteristic when the coil is traversed by currents of normal value long continued, and adjustable means opcratively associated with said armature for effecting instantaneous operation of the armature when a current of abnormal predetermined value traverses the coil.

2. A protective relay comprising an armature, a magnet coil for actuating the armature, a reversibly magnetic member for biasing the armature towards a predetermined position when normally cool and for releasing the armature to said magnet coil when heated to a temperature of a predetermined value in response to normal overload currents long continued, and adjustable means associated with the armature for ellecting instantaneous release thereof, in response to an abnormal overload current of a predetermined value traversing the coil, independently of the thermal condition of said reversibly magnetic member.

A thermal relay comprising a main armature, a magnet coil for actuating the main armature, a reversibly magnetic member for biasing the main armature towards a predetermined position against the influence of the magnet coil when said reversibly magnetic member is cool and for releasing it to the influence of the coil when the reversibly magnetic member is heated to a temperature of a predetermined value, and an auxiliary armature yieldingly mounted on the main armature tor efi'ecting instantaneous release of the main armature responsive to abnormal overload currents of short duration.

4. A thermal relay comprising a main armature, a magnet coil, a reversibly magnetic member connected in series-circuit relation with the magnet coil, said reversibly magnetic member, when at a temperature below a predetermined value, being eiiective to maintain the main armature to a predetermined position against the influence of said coil and to release the same when heated to a ten'iperature above said predetermined value, and an auxiliary armature yieldingly mounted on the main armature for effecting release of the main armature instantaneously when a sudden abnormal overload current traverses the coil and reversibly magnetic member, independently of the temperature thereof.

5. A thermal relay comprising a magnet coil, a main armature, means for e-fiecting movemeni of the main armature responsive to normal currents long continued, and an auxiliary armature yieldingly secured to the main armature for effecting substantially instantaneous IDOYCIHQHt of the main armature responsive to abnormal currents of short (luration.

In testimony whereof, I have hereunto subscribed my name this 27th day of March,

HAROLD E. WHITE. 

